1. Field of Invention
The technical field of the invention is that of prefilled and disposable needleless syringes functioning with a gas generator and used for intradermal, subcutaneous and intramuscular injections of liquid active principle for therapeutic use in human or veterinary medicine.
2. Description of Related Art
For the injection devices according to the invention, a liquid active principle consists of a more or less viscous liquid, or a mixture of liquid, or a gel. The active principle can be a solid dissolved in a suitable solvent for injection. It can also be represented by a powdered solid in more or less concentrated suspension in a suitable liquid. The particle size of the principle must be compatible with the diameter of the conduits in order to avoid blockages.
The needleless syringes according to the invention have the particular feature of functioning with a pyrotechnic gas generator which involves a pyrotechnic charge consisting of the mixture of two powders, the main benefit of this being to control, over the course of time, the pressure of the liquid active principle as it leaves the nozzle, in such a way that each phase of the injection is effected under the required conditions.
In the field of needleless syringes for injection of liquid active principles, it transpires that there is no patent relating to the use of a pyrotechnic gas generator involving the mixture of two powders. By contrast, the use of a single pyrotechnic charge for this type of syringe already exists and is the subject of several patents. By way of example, mention may be made of the patent U.S. Pat. No. 2,322,244 which relates to a hypodermic needleless injector functioning with a blank cartridge. The liquid to be injected, placed in contact with the cartridge, is expelled from the injector under the effect of the pressure generated by the combustion gases. Another patent, WO 98/31409, describes a hypodermic injection system involving a pyrotechnic charge which consists of an explosive or a powder. The specific feature of this injector is that it is designed to attempt to control the problems associated with the kinetics of expulsion of the liquid active principle, not by acting on the characteristics of the pyrotechnic composition, but by having a specific geometry defining in particular an adjoining gas expansion chamber which is provided with a vent. The pyrotechnic charge, which is located in immediate proximity to the liquid active principle, acts directly and instantaneously on said principle by giving it a very high initial speed, while the gases invade the main chamber and the adjoining chamber. The pressure exerted on the active principle then decreases and eventually fixes at an almost constant value, which is sufficient to cause it to penetrate the patient's skin. The adjoining chamber makes it possible to regulate this pressure. Finally, the patent U.S. Pat. No. 2,704,542 relates to a method of injection by liquid jet. This method does not specifically involve a pyrotechnic charge but uses a device intended to control the pressure profiles. In this case, the method used to achieve this objective is based on the two-stage sliding movement of a two-part piston formed by a central cylinder of small cross section housed in a hollow cylinder. An upstream pressure first causes a displacement of low amplitude of the central cylinder in order to communicate a brief but very intense impulse to the liquid which is to be expelled, then the whole of the piston displaces in order to continue to expel said liquid at the appropriate pressure, so as to ensure proper penetration.